In a power generation circuit, there is a DC-DC converter serving as a voltage converting circuit based on a switching system. Herein, the term “switching” connotes switching ON/OFF the switching element such as a FET (Field Effect Transistor) and chopping an input voltage as a square wave. A smoothing circuit smoothes this type of square wave. Then, for example, a desired output voltage value can be obtained by changing a width of the square wave, i.e., a duty ratio.
By the way, when the input voltage is transformed into a pulse wave by this switching element, ringing which accompanies quite a large overshoot occurs depending on an ON-speed (an operation speed, rise time when performing an ON-operation, etc) of the FET. The ringing is defined as a fluctuation component having a shorter cycle than a pulse width of a square wave. The phrase [quite a large] implies “being unignorable” as compared with an amplitude of the pulse waveform.
This ringing becomes a noise source which causes malfunctions in other electronic circuits and a self power circuit inclusive. There are some methods of preventing the ringing.
FIG. 1 illustrates an example of a voltage adjusting circuit having none of countermeasure against the ringing. The voltage adjusting circuit includes a DC-DC controller 1, a pair of FET1, FET2 that receives a control signal from the DC-DC controller 1 and switches an input voltage VIN given from a power source 10, and an inductance element L1 and a capacitor C1 that are supplied with the pulse voltage undergoing the switching by the FET1, FET2 and smooth the pulse voltage in a way that accompanies accumulating and discharging an energy.
The DC-DC controller 1 supplies the pulse voltage alternately to a gate of the FET1 and a gate of the FET2. Then, the DC-DC controller 1 alternately switches ON and OFF the FET1 and the FET2. Both of the inductance element L1 and the capacitor C1 restrain abrupt changes in current and in voltage, and hence a load is supplied with the voltage corresponding to a duty ratio of the pulse waveform based on the switching by the FET1 and the FET2.
FIGS. 2 and 3 illustrate examples of a voltage waveform at a point A in FIG. 1. The point A is defined as a point that is connected to any one of the power source 10 which supplies an input voltage VIN and a ground potential via the FET1 and the FET2. FIG. 3 depicts an example of an ideal voltage waveform at the point A. The original ideal waveform is a waveform approximate to the square wave as in FIG. 3. Normally, a peak value (amplitude) of the square wave is substantially coincident with the input voltage VIN.
On the other hand, FIG. 2 illustrates an example of a result of observing the voltage at the point A in FIG. 1. As in FIG. 2, when observing the waveform at the point A, a fluctuation component having a faster cycle than a switching frequency is detected from a higher voltage than the input voltage VIN as the case may be. This fluctuation component takes a spike-like shape that is shorter in cycle than the square wave in many cases and is called a ringing noise. The ringing noise occurs at the cycle and with the amplitude which depend on characteristics of the respective circuits, corresponding to a switching speed of each of the FET1 and the FET2, a stray capacitance of the terminal and an inductance of the inductance element L1, etc. The ringing noise is superposed on the square wave and forms the voltage waveform containing a higher peak value than the original amplitude value VIN of the square wave.
[Patent document 1]
    Japanese Laid-Open Patent Publication No. 2000-209857[Patent document 2]    Japanese Laid-Open Patent Publication No. 05-3671[Patent document 3]    Japanese Patent No. 3370065